PROJECT SUMMARY/ABSTRACT As witnessed in the 2009 H1N1 pandemic and, more recently, in the severe flu season of 2017-2018, the current strategy of seasonal influenza vaccination based on strain-specific immunity is inadequate. This major limitation has forced NIAID to prioritize the development of a universal influenza vaccine. Toward this goal, we examined the role of type II innate lymphoid cells (ILC2s) and our preliminary data indicate that activated ILC2s can significantly augment the efficacy of commercially available inactivated influenza vaccine, Fluzone. Specifically, we have found that co-administration of ILC2 activating cytokine interleukin (IL)-33 significantly increases both the strain-specific and cross-protective efficacy of Fluzone. Using gene knockout mice, it was established that the adjuvant effect of IL-33 is ILC2 and intranasal route dependent. However, the details of the immune mechanisms governing how ILC2s enhance vaccine efficacy are not known. We hypothesize that IL-33 signaling upregulates ILC2 functions that are critical for induction of cross-protective humoral and cellular immunity in the lungs. The following specific aims will test our central hypothesis: specific aim 1) determine the functions of IL- 33 activated ILC2s that enhance mucosal vaccine efficacy; and specific aim 2) determine whether ILC2s regulate vaccine induced adaptive immunity to confer cross-protection. These aims will be investigated using genetically modified mouse strains and a well-established mouse model to evaluate vaccine efficacy against influenza challenge in vivo. The goal of this project is to expand our knowledge on how ILC2 functions can be harnessed to improve the efficacy of mucosal vaccines. Thus, we anticipate that this study will ultimately contribute towards the discovery of effective mucosal vaccination strategies against respiratory pathogens.